In recent years, magnetic cards enjoyed widespread use in a variety of fields. In particular, the magnetic cards find an expanding application as rewritable or prepaid cards in which monetary information is recorded as magnetic bits of information so that the amount of money may be reduced on every use.
In this application, the safety of the card system is fatally lost if magnetic cards are not fully protected against forgery by the alteration of recorded data or counterfeit cards can be easily produced. There is a strong demand for magnetic cards having a protective function of preventing the alteration of the recorded data. To meet the demand, a variety of magnetic cards have been proposed and used in practice. For example, if magnetic cards are locally provided with a region of a special material, they are difficult to counterfeit and whether they are false or true can be judged by detecting the special region. Magnetic cards with a complex layer arrangement are also known.
These magnetic cards having the protective function incorporated therein are difficult to forge or to duplicate in numbers. However, there is still a possibility that the monetary information in an exhausted card be restored to the initial information by false alteration, for example, by rewriting the monetary information. One countermeasure is to punch holes in the card in accordance with the number of uses although this raises several problems, such as; punched chips left behind, and repair by refilling the holes is possible. Another possible countermeasure is to record visible data corresponding to the number of uses by thermographic recording. These cards carrying visible data, however, are weak against staining because the visible data must be optically read. The falsification of the record is easy on account of visible data. Another problem is that optical readers are generally expensive.
In view of the above problems, JP-A 77622/1996 proposes a magnetic recording medium comprising an irreversible recording layer using as a magnetic recording material an alloy whose ratio of the saturation magnetization in a crystalline state to the magnetization in an amorphous state is at least 5. This magnetic recording medium has an irreversible recording layer composed of a recording material which undergoes an irreversible change of saturation magnetization upon heating. Although this recording material changes its saturation magnetization upon heating, rewriting is substantially impossible because in order to restore the saturation magnetization to the value prior to heating, the magnetic recording medium must be heated to such an extent that the medium may be deformed or melted. The medium insures high safety.
However, since such an irreversible recording layer permits additional writing despite the prohibited rewriting of data, there is a possibility of false alteration of data by additional writing.